Question: how to measure low frequency PWM via HS input

Hi all

First of all, I am using the following hardware:

• CPU 1510SP-1 PN (6ES7 510-1DJ01-0AB0)
• DI 8x24VDC HS (6ES7 131-6BF00-0DA0)

I would like to use the HS inputs to measure the pulse length of a pulse train. This pulse train varies from 50Hz to 500Hz. The pulse train is generated by a fan. This pulse train frequency depends on the fan speed, which is what I want to know. For example, at 3000RPM the pulse train will have a period of 5ms (200Hz). From there on it's an easy conversion to RPM.

I was hoping to find an easy way (=without programming) to find the period / frequency (either will do). The only possibility I see at the moment is to program and count between a certain time (via a timer independently from the user program) and check how much the value has increased in that period of time (for example count during 100ms). If that function returns for example 20, it means that my fan is rotating at 3000RPM.

Does anyone have any experience in similar cases? The datasheet of the HS module says it's possible to count frequencies up to 10kHz, but I hoped it would just give me the frequency :)... Wishful thinking.

Thanks in advance!

Kind regards

Levi

Hello LeviD,

Well there is not much coding you have to do:

• Select your DI module and make it operating mode DI \Oversampling
• Select channel 0 and turn on hardware interrupt, fr event name choise rising edge.
• Inside hardware interrupt block all you have to do is increase some variable for example ImpulseCounter := ImpulseCounter +1;
• Inside project tree add new cycle organization block for example OB35 with cycle interrupt of 200ms = 0.2s, T_Period = 0.2s
• Inside OB35 make the following:
• SPEED_RPM := 60* (ImpulseCounter - ImpulseCounterPrevious)/(T_Period * N), where N = number of pulses per 1 revolution.
  
• As you can see accuracy is related with the number of pulses per revolution and your measuring time period T_Period. For bigger frequencies accuracy will be good, for lower frequencies accuracy is lower, but that's why you need to choice T_Period in right manners.
• If you lower measuring frequency is 50Hz, I would take at least 10 periods of 50Hz., basically to increase accuracy on lower frequency choice T_Period higher, that shouldn't be a problem.
  

Great, thanks for this insight!

I was looking in the wrong direction, I was going to use the inputs in counting mode.